The present invention relates to a channel control method and apparatus, and particularly, though not exclusively, to a method and apparatus for implementing a radio frequency messaging or paging system.
Paging systems generally include at least one transmitter which receives messages addressed to different paging receivers and transmits each message with data identifying the receiver for which the message is intended. Each paging receiver monitors the transmission channel or channels and, when a message addressed to the paging receiver is identified, the receiver stores and displays the message for the user. The message may be a simple alert, or a numeric or alphanumeric string.
In most terrestrial local paging systems, the receiver or pager is tuned to a single frequency channel which is fixed during manufacture, and messages addressed to the receiver are transmitted on this channel. The capacity of such systems is limited by the bandwidth of the single frequency channel.
The capacity may be increased by using more than one transmit frequency and allocating one of these frequencies to each receiver.
The European Radio Messaging System (ERMES), defined by the European Telecommunication Standard 300 133, allows each paging receiver to monitor 16 different frequency channels in turn according to a predetermined schedule, to detect messages addressed to the receiver on any one of these channels. The ERMES system allows the user to roam between different networks and countries in which different frequency channels may need to be used for transmission of paging messages.
However, neither of the above methods is suitable for messaging or paging systems in which bandwidth usage and transmit power must be kept to a minimum such as, for example, a satellite-based messaging system. Moreover, these methods are inflexible as they do not easily allow reallocation of frequency channels, such as adding a new frequency channel to allow an increase in traffic, discontinuing use of a frequency channel when traffic is reduced, or changing frequency channel allocations according to operational constraints.
In many types of radio frequency communication system, channel assignment information is transmitted on a common channel, to which receivers are normally tuned. When a receiver receives a command on the common channel, it retunes to another channel to receive or transmit a message. While this method is suitable for a communication system in which a different traffic channel may need to be used by a receiver for each message, it is not well suited to paging or messaging systems in which the re-allocation of frequencies to receivers is only required occasionally.
The document U.S. Pat. No. 4,849,750 discloses a paging receiver which is responsive to received commands to retune to different message channels.
The document GB 2,284,496A discloses a paging system in which base stations receive and retransmit paging messages. A first set of the base stations retransmits promptly following receipt, while a second set delays retransmission so as not to retransmit simultaneously with the first set.
The document U.S. Pat. No. 5,206,855 discloses a multiple frequency messaging system in which the same paging channel is offset by one slot from one frequency to the next, so that a receiver can receive a message by sequentially scanning the frequencies.
According to one aspect of the present invention, there is provided a paging system in which paging messages are transmitted on a first traffic channel and system control information is transmitted on a control channel. In order to alert paging receivers to new system control information, change indication information is transmitted on the first traffic channel. In response to the change indication information, at least some of the paging receivers tuned to the first traffic channel retune to the control channel and, in response to the new system control information, retune to a second traffic channel. In this way, receivers may be tuned for the majority of the time to traffic channels on which messages can be received, and may be reassigned to different traffic channels. The receivers only need retune to the control channel when traffic channels are reassigned.
Preferably, the change indication information is transmitted more than once before the receivers are required to retune to the control channel, so as to reduce the possibility that some of the receivers may not receive the change indication information because of interference. Preferably, each repetition of the change information indicates the period remaining until the receivers should retune to the control channel, so that the retuning of the receivers may be synchronised even if the receivers do not receive all of the change indication signals.
According to another aspect of the present invention, there is provided a communication system in which first and second traffic channels are transmitted to a plurality of receivers, the first and second traffic channels including messages addressed to selected ones of the receivers. Signals in the first and second traffic channels are transmitted in a periodic frame structure with any messages being transmitted sequentially within the frame. The beginning of the second traffic frame is delayed by a period less than the frame period with respect to the beginning of the first traffic frame. In this way, the peak transmission power can be reduced.
Where the traffic frames are transmitted via satellite, the above aspect is particularly advantageous, since the peak power output of a satellite is limited by the satellite power sources.
In another aspect, a control channel is transmitted together with first and second traffic channels each having a periodic frame structure with the second traffic channel frames delayed relative to the first.
The control channel includes first and second traffic channel data indicating properties of the first and second traffic channel respectively, transmitted sequentially. The order of transmission of the first and second traffic channel data is the same as the order of the beginning of the first and second traffic frames. Thus, the interval between receiving traffic channel data on the control channel and tuning to the corresponding traffic frame is substantially the same for all traffic channels and may be set as short as possible while still ensuring that all receivers are able to retune to their respective traffic channels before the beginning of the next traffic frame.
The present invention extends to components of the above systems and methods performed thereby.